1. Field of the Invention
The present invention relates to subscriber stations, and is more particularly concerned with analog/digital subscriber stations.
2. Description of the Prior Art
In order to provide communication between two telecommunication stations, transmission devices are required between the stations which make a sufficiently noise-free signal transmission possible in both directions of transmission. To this end, for each direction of transmission there can be provided a separate transmission line which in the case of multiplex operation is, multiply exploited for a plurality of simultaneously-existing connections of two respective telecommunication stations. Such four-wire operation is preferably provided in high telecommunication network levels. In lower network levels, particularly in the area of the subscriber branch lines, signal transmission in duplex operation over two-wire lines is generally carried out. For the purpose of decoupling the transmission signals of the two transmission directions, a signal transmission in the discrete position method can be provided in that transmission channels which are separated from one another, either chronologically and/or in frequency position, are provided for the separate directions of transmission, and/or the separation of the transmission signals of the two transmission directions can be effected with the assistance of a hybrid in the form of a bridge circuit terminating the two-wire line, the hybrid transferring, for example, the two-wire line into the four-wire part of a subscriber terminal, and vice-versa. In order to achieve a complete decoupling of the receiving branch of the four-wire line outgoing from the hybrid from the transmission branch of the four-wire line incoming to the hybrid, the bridge circuit must be balanced, to which end it must contain a precise simulation of the input impedance of the two-wire line. In practice, the analog signal hybrids transferring the respective two-wire branch line into a four-wire voice current/audio current line branch which are presently provided at analog telephone subscriber stations which are standard today in the analog telephone network, contain a compromise simulation of the (frequency dependent) line impedance with sufficient accuracy only in a relatively narrow frequency range. This, however, is acceptable for analog telephone traffic.
Recent developments in telecommunications technology have lead to digital telecommunication systems which provide a conversion of the voice signals into digital signals, and vice-versa, for the telephone subscriber in the subscriber station and in which digital telephone connections can be completed by way of digital four-wire switching centers in uniform communication channels with a bit rate of, preferably, 64 kbit/s (per transmission direction), whereby a signaling connection for so-called "out-slot" signaling can constantly consist out of ("out-slot") the 64 kbit/s communication channel in an additional signal channel (subscriber signal channel) with a bit rate of, for example, 8 kbit/s (per transmission direction) (ISS'79, 773, 777; telcom report 2 (1979) 4, 254, 259). In addition to speech, text, data and images can also be transmitted; such a transmission and switching of the digital signals is to be expected in a future integrated services telephone network (ISDN).
At present, and in the near future, the two-wire switching centers standard today in the analog telephone network are employed, in the meantime, both for local switching centers and for private branch exchange (PBX) systems, analog telephone subscriber stations provided with an analog hybrid terminating the two-wire line branch being connected to the two-wire switching centers via two-wire branch lines respectively having a signaling circuit in the two-wire line branch. Upon introduction and further perfection of an integrated services digital network, a large number of such telecommunication systems will therefore already be in operation in the framework of the traditional analog telecommunication network, and the present invention discloses a way to be able to expediently exploit the advantages of digital communication methods.
In this context, it is already known (NTF 73, 1980, pp. 36-40) to superpose a 96 kbit/s digital communication channel lying above the telephone band upon the analog telephone channel of a telecommunication subscriber station connected to a two-wire branch line, in that the two-wire branch line is terminated by a diplexer to whose low pass filter branch the standard analog telephone device is connected and to whose high pass branch a digital transmitting/receiving circuit is connected via an adaptive hybrid in the form of a bridge circuit connected to an echo compensator. Such a telecommunication subscriber station seems relatively involved insofar as, in addition to a diplexer operating as a frequency multiplexer/demultiplexer, a respective hybrid to be realized by a bridge circuit must also be provided, both in the low pass branch and in the high pass branch, whereby the bridge circuit in the high pass branch which is too narrow-banded per se must be augmented by an echo compensator.